PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP5W30O110]14–, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

a Calculated using the PLATON routine SQUEEZE 1 b Calculated based on a PEG-400 density of 1.125 g/ml. This would correspond to a molecular radius of 5.2 Å, which is within the range of expected hydrodynamic radii of PEG-400. 2 Figure S3. Powder X-ray diffraction patterns of PEG-infiltrated {P5W30} crystals synthesized with (i) Mn, (ii) Fe, (iii) Ni and (iv) Zn, which all resulted in isostructural Immm frameworks (Table S1). Crystals synthesized with (vi) Cu resulted in crystals of unbridged, decorated clusters ( Figure S5).

Experimental Methods
Chemicals. Chemicals were purchased from the manufacturers provided in Table S6 and used without further purification.

Synthesis of the Preyssler cluster K14-xNax[NaP5W30O110]•15H2O ({P5W30}).
{P5W30} was synthesized following previously reported methods. 8-11 Na2WO4·2H2O (9.90 g, 27 mmol), 85% H3PO4, (7.0 ml), NaCl (1.17g, 20 mmol) and H2O (21 ml) were added to a 43-ml Teflon-lined acid-digestion vessel. The mixture was stirred until all solids were dissolved at room-temperature, after which the closed vessel was placed in an oven at 125 °C for 20 h. After the reaction was cooled to room-temperature KCl (3 g, 40 mmol) was added and the pale-yellow precipitate was collected by filtration. The product was recrystallized 3 times with 10 ml hot DI water. The final white crystals were collected by filtration and dried for 3 h using vacuum filtration and the purity was verified by collecting a 31 P-NMR spectrum in D2O.
To a 25 ml round-bottom flask was added 7.0 ml 1 M LiCl (7.0 ml, 1 M, pH 1.0, adjusted by 6 M aqueous HCl), CoCl2•6H2O (156.0 mg, 0.656 mmol) and K12.5Na1.5[NaP5W30O110]•15H2O (130.0 mg, 0.015 mmol). The flask was equipped with a reflux condenser and the resulting pink solution was heated with rapid stirring to 90 °C for 12 h. After transferring the solution to a 20 ml vial, the solution was concentrated on a hot plate at 80 °C to a volume of ~1.5 ml (~3 h). After the solution was cooled to room temperature polyethylene glycol (PEG, MW ~ 400, 780.0 mg, 1.95 mmol) was added. If precipitates formed upon PEG addition, the solution is not fully cooled. Adjust the solution by one S12 drop of 1 M LiCl or until the mother liquor is pink. The vial was placed into a sealed 60-ml jar containing 20 ml MeOH. Pink crystals were observed after 3 days and were collected after 2 weeks. Crystals were washed 6 times with 10  Following complete characterization of the single-crystal samples, a "rapid" method was used to synthesize larger sample batches of Co-PEG-Immm in less time. To a 500 ml round-bottom flask was added 2.

Formula Determination
For all the materials presented above, Li, K, M (M = Mn, Fe, Co, Ni, Cu or Zn), Na, P and W contents were determined using inductively coupled plasma mass spectrometry (ICP-MS) and the total mass % of H2O + PEG was determined using the % mass lost during thermogravimetric analysis (TGA, 30-600 °C). For Co-PEG-Immm and Preyssler-PEG-C2/m, the % PEG was determined using C/H/N combustion and was corroborated using quantitative NMR spectroscopy. Water was assumed to be the remainder of the mass lost during TGA. Water was assumed to be the remainder of the mass lost during TGA. Further details are provided in the Characterization Section.

Formation of Films and Gels
Film and gel formation is outlined in Figure S12 Films. Co-PEG-Immm (60 mg) was dissolved in 1000 μl H2O and 200 μl of this solution was dispensed on a 96-well culture plate lid. Water was allowed to evaporate and free-standing films formed after ~12 h. The mechanical properties of films formed this way are dependent on the humidity during evaporation. Films formed when the relative humidity (RH) is below ~60% are rigid/brittle and those formed above RH ~60% are flexible. The films could be switched between flexible and rigid states by placing in a controlled humidity environment or by driving off water using heat. This allowed switching within ~1 min.
Gels. Co-PEG-Immm (1 g) was dissolved in 100 μl H2O and the solution was heated at 50 °C for ~3 min to promote full dissolution.

Characterization
Basic characterization and formula determination. Infrared absorption spectra were collected using an Agilent Cary 630 ATR-FTIR. NMR spectra used to confirm cluster purity were collected using a Bruker AVA spectrometer (300 MHz).
ICP-MS was collected using a Thermo iCAP RQ ICP-MS. The samples were digested in a 1:1 mixture of 85% HNO3 (Optima grade, Sigma Aldrich) and 30 % H2O2 (Trace metal grade, Sigma Aldrich) with sonication for 90 min. TGA was collected using a Perkin-Elmer STA 6000 under N2 with a heating-rate of 5 °C/s. Representative TGA data are provided in Figure S13. C/H/N analysis was conducted by NuMega Resonance Labs using a Perkin Elmer PE2400-Series II, CHNS/O. For quantitative NMR measurements, Co-PEG-Immm crystals (20 mg) were dissolved in ∼700 μl D2O with an internal standard of DMSO (20 mg). Spectra were collected at room-temperature on a Varian VX 500 Spectrometer with an inverse gated decoupling pulse sequence to remove NOE signal (delay time = 360 s). The resonance of the repeating CH2 of PEG (δ = 69.41 ppm) was integrated from 69.783 to 68.883 ppm. These experiments yielded 11.67 % mass PEG (compared to 12.24% by C/H/N combustion).
Single-crystal X-ray diffraction. Single-crystal X-ray diffraction was performed using a Bruker APEX-II Ultra CCD diffractometer equipped with Mo Kα radiation (λ= 0.71073 Å). Crystals were mounted on a Cryoloop with Fomblin Y oil. Data were collected in a nitrogen gas stream at 100(2) K using Φ and ω scans. Crystal-to-detector distance was 45 mm and exposure time was 10 s per frame using a scan width of 0.75°. Indexing and unit cell refinement indicated a primitive, orthorhombic lattice. The data were integrated using the Bruker SAINT software program and scaled using the SADABS software program. Solution by direct methods (SHELXT) produced a complete phasing model consistent with the proposed structure. All nonhydrogen atoms were refined anisotropically by full-matrix least-squares (SHELXL-2014). 12 For highly disordered water molecules, the PLATON routine SQUEEZE was used to account for the corresponding electrons as a diffuse contribution to the overall scattering without specific atom positions. 1 In order to model disorder structures ISOR, EADP and RIGU commands have been used. For Mn-PEG-Immm, Fe-PEG-Immm, Co-PEG-Immm, Ni-PEG-Immm and Zn-PEG-Immm, the position of all countercations have not been assigned due to high disorder. All Immm deposited structures and PEG-{P5W30} contain two disordered configurations of the inner tungsten belt of S16 {P5W30}. This disorder is removed to show a simplified structure in Figures 1a and S1. PLATON's CALC SOLV 1 was performed on Co-PEG-Immm and Preyssler-PEG-C2/m to find the total accessible solvent in the void volume percent.
The Ni-PEG-Immm data has not been deposited to the CCDC/FIZ Karlsruhe database. Although the structure is isomorphous with the other PEG-Immm structures, Ni-PEG-Immm has high diffuse scattering and thermal motion, in which classical refinement values have a high Rvalue of ~16%. A simple disorder model did not improve the structural solution. Three full data sets were screened and yielded similar diffuse scattering with motion along the b direction due to hydrogen-bonding in the structure, the soft serial arrangement of water, and the presence of PEG. The cluster shows two possible disordered confirmations, one of which is rotated and shifted, most likely due to the flexibility of the Ni bridging ion. Furthermore, attempts at lowering the symmetry did not improve the disorder model. The structure was thus not deposited to database, but the CIF is provided as supplemental information.
In Cu-PEG-C2221, Cu1 is disordered and modeled with two positions at half occupancy. Cu1A is coordinated by one {P5W30}, while Cu1B is not coordinated by {P5W30}. Although only 4 water molecules could be assigned near Cu1, the blue color of the framework indicates octahedral coordination of Cu 2+ . It is thus assumed that Cu1A and Cu1B share some disordered water in their environment.
Powder X-ray diffraction. For powder X-ray diffraction measurements, crystals were crushed between two glass slides and placed on a mounting loop. Data were collected on a Bruker K3 Kappa Vantec 500 diffractometer equipped with Cu Kα radiation (λ = 1.54184). The measurements were performed in transmission mode with detector distance of 200 mm and 3 frames collected φ = 0° and 45°. The first, second, and third frames were centered at 2θ = 10°, 22° and 34°, respectively. The diffraction rings obtained from the three frames were overlaid together and radially integrated in Diffrac.EVA V.4.2.2 (Bruker).
Scanning Transmission Electron microscopy (STEM). A rigid film of Co-PEG-Immm was sonicated in ethanol and the suspension was pipetted onto a lacey carbon grid. Medium angle annular dark field (MAADF) images were acquired using JEOL Grand ARM aberration-corrected STEM under 300kV. The enhanced image was generated after noise-filtering the fast Fourier transform of a selected area of the original image.
Scanning Electron Microscopy (SEM). A rigid film of Co-PEG-Immm was affixed to the SEM sample holder using carbon tape. Images were collected on a FEI Apreo SEM with an operating voltage of 5.00 kV and emission current of 0.80-0.10 nA.
Rheology. Rheological measurements conducted using a Discovery HR-3 rheometer (TA instruments). Samples were prepared fresh and measured within 1 hr. The gel was placed on the surface of a 40-mm sandblasted plate and with a loading gap of 45000 µm. The temperature was set at 20 °C using a Peltier heating system. For oscillatory tests, an amplitude sweep was carried out at strains of 1.0-100 % with an angular frequency of 10 rad/s. The frequency sweep was made from the linear region of the amplitude state from the oscillatory test to determine the linear viscoelastic range. The frequency sweep was performed at 2.6 % strain at 20.0-300 rad/s. Raman spectroscopy. Raman spectra were collected using a Thermo Scientific™ DXR™3 SmartRaman with an extended gradient of 6000 to 50 cm −1 . Using a YAG laser that had singlemode operation at 532 nm with an output power of 10 mW, the spectra were measured over the frequency range between 50 and 4000 cm −1 . The laser beam was focused using an optical system and open beam expanded onto the surface of the sample at a spot with a 50 µm diameter.